<?php
echo $this->Ajax->observeField('UserDepartamentId', array(
    'url' => array('controller' => 'users', 'action' => 'select_city'),
    'frequency' => 0.2,
    'update' => 'ciudad'
        )
);
echo $this->Form->input("User.city_id", array('label' => 'Municipio', 'empty' => 'Municipio', 'options' => $cities, 'class' => 'required'));
?>
<div id="ciudad">

</div>